A logic gate is generally used for logical operations in data processing using one more logic inputs to produce a single output. The logic is normally referred to Boolean logic such as NOT, OR, NOR, XOR, XNOR, AND, and NAND. For an electronic central processing unit (CPU) the logic gate acts as a basic unit to perform computing operations. Due to the increasing demand for faster processing ability, the CPU has been exponentially evolved up to now 3.7 GHz in 2006 according to the Moore's law since mid 1970s. However, there have been fundamental limitations in electronic transistors in such a way of gate-width dependent switching time and electromagnetic-interference-limited operational bandwidth. To overcome such fundamental limitations, optical logic gates have been studied for last several decades. On the contrary, as a counterpart of electronic carriers, optical signals are free from the electromagnetic interference and the size-dependent speed limitation. Moreover parallel processing is an intrinsic benefit. Recently, various types of high-speed all-aptical logic gates for Boolean NOT, OR, NOR, AND, XOR, and NAND have been demonstrated using semiconductor-optical-amplifiers (SOAs) and erbium doped fiber amplifiers (EDFAs). A computing device, however, must satisfy certain requirement of practicality such as lower power consumption, smaller device size, and higher speed. Since 1980's several types of Boolean algebra-based optical logic gates have been proposed and demonstrated. A bulky optical logic gate using linear optics such as mirrors and beam splitters has been applied. The linear optics based optical gate is of course lacks efficiency due to bulky size.
Recently nonlinear optics-based optical logic gates have been intensively studied. In this area, SOA is the most useful component to form the optical logic gate. By introducing SOAs to optical logic gates the device size and power consumption has been extremely reduced down. The physics of the SOA-based optical logic gate is using refractive index change occurred by electric current. In the conventional optical switching technologies, the time needed for the refractive index change is an absolute constraint to the switching time. Here, the refractive index change is limited by the carriers' redistribution time. Therefore, the conventional optical switching time should be fundamentally limited by the carriers' lifetime, where it is sub-nanosecond. Even though a SOA offers relatively low power consumption such as ˜100 mW (Optics Letters, Vol. 23, pp. 1271-3 (1997)), total estimated power and size for a potential optical CPU composed of just million units of SOA reaches at several ˜100 kW and several square meters, respectively, which is never practical. Here it should be noted that the up-to-date electronic CPU reaches at 3.7 GHz in clock speed, contains near 300 million transistors, and needs about 100 W power consumption: www.intel.com/research/silicon/micron.htm
On the other hand, optical switching effect can be obtained by using a nonlinear quantum phenomenon, electromagnetically induced transparency (EIT), which uses two-color electromagnetic fields for rapid refractive index change owing to quantum interference in an optically resonant medium composed of three energy levels or more: Harris, Physics Today, Vol. 50, p. 36 (1995). The energy level structure of the resonant optical medium satisfies two-closely spaced ground states and an excited state, two-closely spaced excited states and a ground state, or an arbitrarily spaced cascade-type system. The quantum interference-based refractive index change can result in strong spin coherence excitation on the closely spaced states and absorption cancellation. Due to the abrupt absorption spectrum change, a slow light phenomenon is induced due to the steep dispersion slope across the resonance frequency: Turukhin et al., Physical Review Letters, Vol. 88, p 023602 (2002).
In the case of EIT, the time needed for the refractive index change is not limited by the carriers' lifetime or population relaxation time, but dependent on the phase decay time, where the phase decay time is normally much shorter than the population relaxation time in solids. Specifically, the phase decay time is hundreds times faster than the carrier's lifetime in most ion-doped crystals such as Pr 3-doped Y2SiO5, so that ultrahigh-speed optical processing can be obtained: quantum switching (Ham, Applied Physics Letters, Vol. 85, pp. 893-5 (2004)). The two-photon coherence excitation between the closely spaced ground states is optically detected by using nondegenerate four-wave mixing processes. The resulting intensity of the nondegenerate four-wave mixing signal can be stronger than that of the original input signal: Hemmer et al., Optics Letters, Vol. 20, pp. 982-4 (1995)).
When the two-closely-spaced ground states are replaced by three-closely-spaced states and three-color optical fields are resonantly applied to the common excited state, a quantum switching phenomenon is obtained: Ham, Physical Review Letters, Vol. 84, pp. 4080-3 (2000). The physics of the quantum switch is the optically controllable spin-coherence swapping, and the optical switching time is much faster than conventional refractive index-based optical switch. This fact was experimentally demonstrated using a Pr3+-doped Y2SiO5 (Pr:YSO) for 100 fold decreased optical switching time: Ham, Applied Physics Letters, Vol. 85, pp. 893-5 (2004).